Use isSingleContext() helper

Author: eregon

src/main/java/org/truffleruby/language/methods/CallInternalMethodNode.java

@@ -140,7 +140,7 @@ static Object[] packArguments(Object callerData, InternalMethod method, Object s
     }
 
     protected Assumption getMethodAssumption(InternalMethod method) {
-        return RubyLanguage.getCurrentLanguage().singleContext
+        return isSingleContext()
                 ? method.getDeclaringModule().fields.getOrCreateMethodAssumption(method.getName())
                 : AlwaysValidAssumption.INSTANCE;
     }